1. Field of the Invention
This invention relates to a liquid crystal display including a nematic liquid crystal with positive dielectric anisotropy and twisted configuration and having as well chiral addition, and at least a polarizer, wherein the support plates of the display cell exhibit orientation layers, of which at least one produces in the adjacent liquid crystal layer a tilt angle of more than 5.degree..
2. Discussion of the Background
Such a display is known from J. App. Phys. 53, December 1982, 8599-8606 (corresponding to GB-A 2,087,583). The liquid crystal in the known display is twisted 360.degree. and the tilt angle is about 35.degree.. By the great twist a bistability effect is achieved so that for an applied holding voltage two differently twisted liquid crystal configurations exist. Switching from one configuration to the other occurs by momentary raising or lowering of the withstand voltage. Changing of the optical properties is made visible by means of crossed polarizers by use of the Schadt-Helfrich effect (Appl. Phys. Lett. 1B (1971) 127), i.e., the orientation directions of the orientation layers are either parallel or perpendicular to the vibrational direction of the polarizers, and the product of liquid crystal layer thickness d and birefringence .DELTA.n is great in comparison with the wavelength of light. The chiral doping is so selected that a ratio of layer thickness d to pitch p of the twist of about 0.98 results. The main interest is in range 0.95 to 1.10. Above 1.15 the bistability disappears and below 0.95 the switching times become too long. Layer thickness d is 15 .mu.m. The measurements described in the publication go to d/p=0.9. Also a value of d/p=0.750 is mentioned, and a cell with d=7 .mu.m is also considered. This known display is controlled so that it is written by the line. Since it must be continuously renewed, only a few lines can be written. This means that the degree of multiplexing is low and a large dot matrix display is not achievable.
In U.S. Pat. No. 4,143,947 a liquid crystal display is described in which the decay time after an electric activation is to be reduced. The liquid crystal exhibits a twisted configuration, which is caused by orientation layers rotated 90.degree. in relation to one another on the cell plates. A chiral doping is provided so that a natural pitch p between d and 4d (d=layer thickness) results. As a result, twist angles of 90.degree. or 270.degree. are formed. The dielectric anisotropy is positive, and the change of the optical properties with the application of a voltage is made visible by the Schadt-Helfrich effect. The liquid crystal layer thickness is 12.5 .mu.m. The described display exhibits no advantages in regard to multiplexing capability in comparison with conventional liquid crystal displays of the Schadt-Helfrich type. They thus especially have a relatively flat characteristic curve and a strong viewing angle dependence. Therefore it is not suitable for large displays with high information density.
Finally, from JP-A-57 133 438 a liquid crystlal display is known which exhibits a twist angle of 270.degree.. The optical changes with the application of a voltage are made visible by the "guest-host effect" (Heilmeier et al. in "Molecular Crystals and Liquid Crystals" 1969, pp 293-304 and/or Appl. Phys. Lett. Aug. 1, 1968, 91,92), i.e., the liquid crystal is doped with a dichroic dye, and the cell is provided with an input polarizer, whose vibrational direction is parallel to the orientation layer. This known display is also unsuitable for high degrees of multiplexing, i.e., because of too low a contrast.